Mast tube design

A common antenna tower configuration is a lattice tower structure
with a tube mast out of the top of the tower to support antennas. The
tube mast is usually rotatable, cantilevered and supported in bearings
to resist the lateral and thrust forces. This article discusses briefly
the design of that mast, and offers a spreadsheet design support tool.
The article does not consider the implications for loading of the tower
structure itself.

A range of materials are available for the purpose, but for most
high performance applications, performance Aluminium or Steel alloys
will be best suited. Not only are these tubes expensive, but transport
costs can be even greater, especially in longer lengths.

The figure above show s a range of solutions for 50mm OD and 60mmOD
tubes. The notation is the <product>
<OD>x<WallThickness>.

The second bar is common 40mmNB water pipe, and shows it is a low
strength option.

The VK2KRR option is his recommended sleeving of an aluminium tube
with a high performance steel tube, being the first and third tubes
shown above. It should be noted that unless the sleeve is effectively
prevented from sliding at all under the highest safe bending moments,
it should be treated as a sliding interface and the maximum bending
moment of the combination is no greater than the sum of the maximum
bending moment of each of the tubes. That is the main reason that the
Steel 4130 50.8×4.77 option has much higher strength than the VK2KRR
composite even the the latter is much thicker.

Attached is a spreadsheet design tool to assist with calculations.

Above is an upper section which describes one of the antennas
installed on the mast.

Above is the summary section where all of the component bending
moments are summed and the wind and tilt stresses calculated.

In this case, a Safety Factor of 1 is used for the wind
calculations, so that the mast is likely to bend before tower failure,
though that will not necessarily save the tower. The Safety Factor for
tilt is 3 to make provision for, amongst other things, dynamic forces
as the tower is raised or lowered.

Design wind speeds appropriate for location and environment need to
be selected.

In this example, the required yield strength is within the strength
of 6061 Aluminium alloy, and a relatively low cost tube that fits the
standard 50mm opening in common towers will suffice. Grab the MastCalculations
spreadsheet and try raising the antenna, or adding another antenna and
explore the results.

The spreadsheet makes provision for up to three antennas, cable,
mast, and rotator above the top of the mast. Obviously, if the rotator
is inside the tower, it does not contribute to the bending moment of
the mast tube.

Warning, the spreadsheet does
not replace the need for design competence. It is only a support tool,
you MUST NOT depend on its results without
independent verification. NO RESPONSIBILITY IS ACCEPTED NOR
WARRANTY OF ANY KIND OFFERED.